1. Field of the Invention
The present invention relates generally to a miniature X-ray device. More specifically, the present invention relates to an X-ray device for a catheter. More specifically, the present invention relates to an X-ray device having a focusing cup containing an electron emitter material.
2. Background Art
Cardiovascular diseases affect millions of people, often causing heart attacks and death. One common aspect of many cardiovascular diseases is stenosis, or the thickening of an artery or vein wall, decreasing blood flow through the vessel. Angioplasty procedures have been developed to reopen clogged arteries without resorting to a bypass operation. However, in a large percentage of cases, arteries become occluded again after an angioplasty procedure. This recurrent thickening of the vessel wall is known as restenosis. Restenosis frequently requires a second angioplasty and eventual bypass surgery. Bypass surgery is very stressful on the patient, requiring the chest to be opened, and presents risks from infection, anesthesia, and heart failure.
One method of treating restenosis includes using miniature X-ray devices to irradiate blood vessels and other human body cavities. An X-ray catheter is comprised of a coaxial cable and a miniature X-ray emitter connected to the cable""s distal end. The proximal end of the coaxial cable is connected to a high voltage power source. The X-ray emitter consists of an anode and a cathode assembly mounted in a miniature shell (tube), made of an insulator with very high dielectric strength. Typically, the anode is comprised of platinum, tungsten, or another heavy metal.
To activate the emitter, high voltage is applied between electrodes. A high electric field is generated at the cathode surface and causes field emission of electrons. Emitted electrons are accelerated by the electric field and impinge on the anode. As the electrons strike the anode, X-ray energy is produced and radiated. The radiation occurs as high-speed electrons are slowed or stopped by passing near the positively charged nuclei of the anode material, or, as incoming electrons collide with anode atoms, knocking the electrons near the anode atom nuclei out of orbit and replacing the knocked out electrons with other electrons.
For coronary applications, the outer diameter of an X-ray emitter must be as small as 1.00 to 1.25 mm. The close proximity of each component of the emitter makes it difficult to control with preciseness the direction of flow of emitted electrons. A fraction of electrons emitted by the cathode may hit the inner wall of the insulating shell. The inner surface of the shell has an electric field parallel to its surface that is capable of creating an electron avalanche along the inner surface. By knocking electrons from the wall, the avalanche causes positive charging of the surface which distorts the electric field near the cathode, causing even more electrons to be emitted to the wall.
An electron avalanche also increases leakage current. Leakage current is electron flow that passes from the cathode to any portion of the emitter other than the anode. When electrons collide with the interior wall of the insulating shell, the electrons provide a path for leakage current along the wall between the cathode and the anode. This leakage current can be hundreds of times as high as the initial field emission current onto the wall. Leakage current does not produce X-rays but causes undesired heating of the emitter.
Additionally, leakage current affects the proper monitoring and calculation of the irradiation dose. Irradiation dose is calculated on the basis of the emitter""s current. As leakage current increases, the amount of X-ray radiation decreases with no change in the measured current. Thus, the measurement of irradiation dose becomes inaccurate and guesswork.
Secondary electron emission (SEE) ratio (or yield) is the number of secondary electrons emitted on the average per incident primary electron. SEE yield xcex4 depends on the energy of the incident electrons. For most insulating materials, and in the range of energy between ten and several hundreds of electron-volts, xcex4 can have a value of about 1 to 5. These insulating materials support electron avalanches at their surfaces which result in positive charging of the wall. However, some materials have an SEE yield less than 1. Accordingly, electron bombardment causes negative charging of the surface and subsequent repelling of the incident electrons, eliminating electron impact to the wall.
What is needed is a miniature X-ray device for an X-ray catheter having a lower incidence of electron avalanche and leakage current.
The present invention is a miniature X-ray emission device having an insulating shell that houses a cathode and an anode. The cathode includes a focusing cup formed into one end. The focusing cup is an axially symmetrically hollow that provides a converging electron beam toward the anode. The anode has a flat surface for collecting electrons emitted from the anode. The flat surface aids in achieving a more uniform azimuthal distribution of this radiated X-ray energy.
The focusing cup can include a thin metal layer that conforms to an inner surface of the cathode. Such a coating avoids the problem of field emission from the edge of the anode, which may be located in moderately close proximity to the wall of the insulating shell. The metal layer is a non-emitting metal of high work function. The depth of the focusing cup is selected such that it is within an area having an electric field in the range of 3-5 times lower than the electric field at the edge of the focusing cup.
An emitting material having a low work function is deposited directly onto the internal surface of the focusing cup. A preferred emitting material is a carbon product, such as graphite or diamond. Diamond is a low work function material having a negative electron affinity. When diamond is used as the emitting material, the emitting product should be a thin layer or film to allow diamond electrons to be properly replenished from the rear of the emitting material, through the anode. The thin layer can be accomplished through laser deposition of the emitting material. A graphite mixture may be used to provide increased electron replenishment over diamond.
An interior coating can be applied as a circumferential belt on the interior surface of the insulating shell. The interior coating extends lengthwise in the region of the cathode to an anode gap, covering the region of the insulating shell most likely to be subject to stray electrons emitted from the cathode. The interior coating is formed of a negative secondary emission yield material, such as chromium oxide or titanium. The interior coating can also be applied to the whole length of the interior surface of the insulating shell.